Ion beam systems with gridded broad-beam ion sources are used for various surface modification, etching and deposition applications. Ion beam processes that provide a source of charged particles are particularly advantageous compared to other methods for providing direct control of ion energy and flux and angle of incidence to the substrate, and isolation of the substrate from the conditions of the reactor used to generate the etching and or depositing species. Broad-beam ion sources have numerous applications in microelectronics device fabrication. Ion beam equipment is extensively used in the production of high frequency microwave integrated circuits and thin magnetic heads.
An early version of a direct-current (DC) ion source for industrial applications is described in U.S. Pat. No. 3,913,320. This type of ion source was developed originally for space propulsion as disclosed in U.S. Pat. No. 3,156,090. Various modifications of such ion sources have been developed to optimize the efficiency of the ion source and to improve the method of extracting the ions or shaping the beam profile for ion beam etching and deposition applications. See, for example U.S. Pat. No. 4,873,467. Typically, these DC ion sources use a heated cathode configured as either a heated filament or hollow cathode.
Ion sources with filament type cathodes are easier to operate, but require frequent replacement of the filament assembly. Furthermore, the hot filaments rapidly degrade in the plasma state by interaction with gases such as hydrocarbons, oxygen, hydrogen, and fluorinated gases which are useful for thin film deposition. The shortcomings of these kinds of DC ion sources hinder the implementation of ion beam processes in manufacturing processes.
The disadvantages of DC ion sources can be avoided by using radio frequency (RF) charged particle sources which employ high frequency electromagnetic energy for ion generation, such as from microwave energy sources. RF inductively coupled ion sources also were originally developed for space propulsion. An example of an RF ion source that uses an axial RF coil is described by H. W. Loeb, “State of the Art of the RIT-Ion Thrusters and Their Spin-Offs,” (1988). In contrast with DC sources, many RF sources do not have discharge electrodes in direct contact with the plasma.
In many broad-beam ion source designs, a grid assembly with two or more grids is used to extract the ion beam from the ion source and direct it to the target. One general limitation of conventional gridded ion beam sources in practical applications is the formation of electrical shorts between the grids that cause instabilities and limit the operational life of the ion source. These shorts are usually developed due to deposits on the grids formed as a result of material erosion inside of the source and the chamber. Direct deposition on the grids can create needle-like deposits (herein also simply referred to as “needles”) that, although not dead shorts, can locally reduce the effective breakdown voltage, and may cause high voltage arcs or shorts. Accumulation of sputtered material on other surfaces, such as the process chamber shields, may build up to a sufficient thickness of material within the chamber that flakes off and the flakes become lodged between grids, thereby causing dead shorts.
Conventional grid short clearing (also simply referred to as “grid clearing”) techniques have been developed for xenon gridded ion thruster technology used in planetary missions by NASA, such as those described in U.S. Pat. Nos. 6,786,035 and 9,038,364. In a publication on grid clearing for the NSTAR ion propulsion system, a low voltage “grid clear” circuit applies a first energy between grids at a first voltage, and a high voltage grid clear circuit applies a second energy at a second voltage higher than the first voltage; Keith D. Goodfellow et al., “An Experimental and Theoretical Analysis of the Grid Clearing Capability of the NSTAR ion propulsion System,” 35th AIAA/ASME/SAE/ASEE, Joint Propulsion Conference and Exhibit; Jun. 20-24, 1999; Los Angeles, Calif.
Methods for clearing electrical shorts in space-based ion thruster applications must accommodate the associated low pressure (high vacuum) conditions (˜10−9-10−5 Torr). The low voltage grid clear circuit can remove electrical shorts by passing sufficient current through the flakes or other deposits between grids to cause them to sublime, but this approach requires a continuous conductive path. It is ineffective for removing electrical shorts that appear only at the higher voltage operating conditions of the ion source, such as the “needles” and other deposits on the grids that do not create a dead short. Even when the grid assembly is considered to be clear after such a high vacuum clearing operation, if another grid-clearing pulse is delivered, electrical shorts between the grids may reappear due to residual flakes or needles reforming conducting paths on one or both grids. For these reasons, conventional grid-clearing methods often provide for a second energy pulse at higher voltage to be applied to generate an arc plasma that creates a low resistance path through any short circuit to enable a current flow sufficient to clear any residual shorts.
At the low pressure conditions of the ion thrusters for which the conventional grid clearing methods were developed, generation of a high voltage arc plasma requires ignition of a vacuum cathodic arc. The conditions associated with creation of a vacuum cathodic arc include initial conductivity between grids and sufficient energy to ignite the vacuum arc. A vacuum cathodic arc is distinguished by the creation of an ionized plasma composed of material ejected from the negatively charged grid surface, which may include material of the grid itself. Generation of such vacuum cathodic arcs may lead to significant local non-uniform temperature increases and strong grid erosion that is damaging to the grid. The grid erosion caused as a result reduces the grid life time and makes it difficult to clean and rebuild the grid assembly. In some cases, “needles” and other deposits on the grids may not appear as shorts and thus may not be effectively removed under the low pressure (high vacuum) grid clearing operations used for space-based thrusters, but under the higher working pressures of typical industrial applications for materials processing systems (˜10−4-10−3 Torr) these kinds of deposits can still can cause electrical shorting and instability during operation of the ion source.
Aside from the operating pressure, there are many other differences between the operating conditions which make the grid clearing techniques for ion thrusters generally inapplicable to ion sources used in industrial applications for materials processing systems. For example, because ions from an ion thruster are ejected into outer space, grid shorts are generated mainly from deposits of material sputtered at relatively low energy from the interior of the ion source, and are therefore relatively infrequent. Whereas, in an ion beam materials processing system, the ion beam cannot escape from the process chamber, resulting in heavy sputtering at comparatively high ion beam energies, which results in generation of a much larger load of flakes and other debris that can short the grids. Hence, the need for grid clearing may be expected to be much more frequent, and any cumulative damage from the grid clearing operation may be more critical in an ion source than for an ion thruster. In general, industrial applications of gridded ion beam sources require less aggressive grid short clearing techniques in order to reduce the potential for grid damage and to provide improved ion beam source operational life, increased grid assembly service life, and increased number of the grid rebuilds before replacement.
There is a continuing need for methods and apparatus for clearing electrical shorts between ion source grids used in industrial applications for materials processing systems due to deposits on the grids, such as “flakes” or “needles” with no or minimal damage to the grids. Accordingly, it would be desirable to provide an ion beam materials processing system with a gridded ion source having a grid short clearing system capable of providing long operational grid life.